I've mentioned this before, but I recommend you seek out the BBC 6-part TV series "The History of Music" presented by Howard Goodall. It's no longer available on the BBC site but it's accessible on Youtube, though not all in one place. In one of the earlier episodes he talks at length about equal temperament, what it is and why it was revolutionary.

I wouldnt go to that direction if I were you... It's like superstition... If you transpose the same music to a different key - especially if it's only instrumental, it sounds exactly the same to 99% of the people. There are tones of theoris about keys... like C is red and D is yellow, or Am is sad Dm is drammatic etc. etc. but I dont believe it's like that really

bob_32_116: Good advice! I've looked at several YouTubes about equal temperament, including the Goodall piece. I now have a decent idea about what it is, and have seen some oscilloscope illustrations of equal temperament vs. just intonation. I can even sometimes hear the difference between the two.

But I've never tuned or played an instrument, so much of what they say isn't meaningful to me. Also, my ear isn't well-trained enough to distinguish between some of the examples that are provided. Before I drop this quest, I'd like to know a bit more about the "colors" and "moods" that were (or were thought to be) associated with non-equally-tempered scales. After all, I'm trying to figure out why classical composers chose one key over another, and that seems to be one of the reasons.